1. Field of the Invention
Embodiments of the present invention generally relate to a downhole tool, and more particularly to packers.
2. Description of the Related Art
In the oilfield industry packers are employed at different stages and can be generally classified by application, setting method and retrievability. A principal function is to seal an annular area formed between two co-axially disposed tubulars within a wellbore. A packer may seal, for example, an annulus formed between production tubing disposed within wellbore casing. Alternatively, some packers seal an annulus between the outside of a tubular and an unlined borehole. Routine uses of packers include the protection of casing from pressure, both well and stimulation pressures, and protection of the wellbore casing from corrosive fluids. Other common uses may include the isolation of formations or of leaks within wellbore casing, squeezed perforation, or multiple producing zones of a well, thereby preventing migration of fluid or pressure between zones. Packers may also be used to hold kill fluids or treating fluids in the casing annulus.
Packers may be run on wireline (a medium for propagating signals between a surface unit and downhole location), pipe or coiled tubing. In each case, the packer includes a setting mechanism which operates to set a sealing element. The type and operation of the setting mechanism and related sealing element may depend on whether the packer is to be set permanently or temporarily (i.e., to be retrieved at a later time). Conventional packers typically include a sealing element (i.e., an elastomeric element) between upper and lower retaining rings or elements. The sealing element is compressed to radially expand the sealing element outwardly into contact with the well casing therearound, thereby sealing the annulus. Alternatively, the expansion of the sealing element may be accomplished by pumping a fluid into a bladder.
As recoverable petroleum reserves are being found at ever increasing depths, packers are required to operate in environments of corresponding higher temperatures and pressures. Packers typically rely on a series of backup rings and support components to contain the elastomer sealing element and prevent extrusion (i.e., migration of the sealing element beyond the defined containment area). Unfortunately, the higher temperatures associated with deeper subterranean operations soften the elastomer sealing elements and lessen their ability to resist extrusion. With increasing temperatures and pressures, all of the interfaces between the backups and support components become potential extrusion gaps for the sealing element.
A particular operation during which conventional packers often fail is when installing liners. It is common practice to place a packer at the liner lap to provide a mechanically formed seal in addition to the seal created by the cement. The sealing elements of such packers are typically tubular shaped sections of elastomer that are slid over a mandrel. The sealing elements are typically activated by applying a compressive force to radially expand the sealing element outwardly into contact with the well casing, as described above. When pumping cement during liner cementing operations, it is desirable to pump at high rates in order to provide a more effective washing action to clean out wellbore debris and prevent channeling of the cement. These high flow rates can cause a low-pressure zone over the unset sealing element of the packer. In addition, higher temperatures cause the elements to expand and become softer, thereby lessening their stability. Under these conditions, conventional elastomer sealing elements may become unstable and swab off, preventing the cementing operations from being completed as desired and possibly damaging the sealing element.
Another downhole condition which detrimentally effects the operation of a sealing element is the interface between casing and the backup rings designed to contain the sealing element. The casing surface that the backup rings contact is typically a rough rolled surface that may be somewhat irregular. In addition, most conventional backup rings are triangular in shape with one of the legs of the triangle contacting the inner casing surface. The angle of the support pieces that urge the backup rings out is typically between about 45 and 60 degrees with respect to the axial centerline of the packer. The relatively irregular contact surface of the casing combined with the angle of the support pieces provides a modest contact force between the backup and the casing. This contact force is often insufficient to contain the sealing element, particularly at elevated temperatures and pressures.
Therefore, there is a need for packers having sufficient pressure integrity for both liquidity and gas, particularly for various high temperature and/or high pressure environments.